Investigation of Biomass Combustion in Grate Furnaces using CFD

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Abstract

Channelling is an undesirable phenomenon in the fixed-bed combustion of biomass. It is characterised by an uneven air distribution, and thus fuel conversion, throughout the fuel bed. Attempts to mitigate the channelling effect using redesigned grates have resulted in an increase in grate wear. Computational fluid dynamics can provide an insight into these issues. To this end, an unsteady, 2D numerical model for predicting solid fuel combustion under fixed-bed conditions is designed and developed. The commercial CFD code Fluent is used as the basis.
The model is then validated using different existing data for ignition rates and species profiles through a biomass fuel bed.
The model is applied to investigate factors that influence channelling in a randomly packed bed of biomass. These factors include: resistance to flow through the grate, the bed height, flue gas recirculation, and the initial moisture content of the fuel. A high value of grate resistance is confirmed as a key factor to reducing channelling.
The 2D model is extended to three dimensions to capture 3D features within the fuel bed and grate. To reduce the simulation run times, the 3D model is parallelised to run on high performance computers or clusters.
The complete 3D model is employed to examine measures to reduce grate wear. The influences of the shape of the grate, the inlet air flux, and the thickness of the ash layer are investigated. Results indicate that small passages are beneficial, but that low grate porosity and/or large spacing between passages may lead to excessive temperatures of the grate material or potentially carburising conditions at the surface of the grate.

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